I have a route stored in my favourites that takes me home from work along many minor roads, which I use to avoid rush hour traffic. This route, which is about 20 miles long, gives an estimated time of 1 hour 17 minutes! I can only assume that the software assumes an extremely low speed for rural minor roads, which is many cases is not correct. One of these roads in question is indeed a country lane, but it is not particularly narrow, and long and straight, and you can easily maintain a speed of 60mph along its entire length. The route in its entirety takes about 35 minutes, less than half the projected time.
Perhaps the speed settings for minor roads needs to be adjusted?